Comment by CommieBobDole
2 days ago
So this is a really neat technical demo, using spacecraft cold-gas thrusters and guidance hardware/software to do crazy stunt stuff in a car, but what's the relevance to an actual product that can be sold, even as a small-volume exotic? These are pressurized gas thrusters, so either the tanks, valves, and piping take up most of the interior of the car, or it has a low-single-digit number of seconds of thrust before recharge, probably both. Also the tanks for these things tend to use incredibly high pressures and are apt to explode spectacularly in any sort of accident.
This is sort of like demonstrating a car that softens the asphalt in front of it with lasers to get a better grip; difficult, technically impressive, and zero chance that it will ever be allowed on a public road.
Edit: According to this Motor Trend article, it uses a big carbon-fiber pressure vessel at 5500psi, so basically an enormous fragile bomb. And the one that did all the stunts had two of the pressure vessels which would have filled most of the interior.
https://www.motortrend.com/news/tesla-roadster-thrust-system...
> a big carbon-fiber pressure vessel at 5500psi
Other cars have these tanks, but full of flammable fuel, not just air.
Off the top of my head, the honda civic CNG (3600 psi)
https://en.wikipedia.org/wiki/Honda_Civic_GX#Specifications
the toyota marai (10,000 psi hydrogen)
https://en.wikipedia.org/wiki/Toyota_Mirai#High-pressure_hyd...
and probably the other cng or hydrogen cars.
It looks like those tanks are 5-8kg? The Tesla's is apparently 250kg.
Hydrogen is 1/14th the density of air.
I thought carbon fiber wasn't good with pressure, hence OceanGate Titan implosion
oceangate isn't a good comparison.
carbon fiber is great in tension, i.e. a compressed air tank, or COPV
carbon fiber is much less great in compression, i.e. OceanGate
Also the OceanGate construction process was bad, even for carbon fiber, because the CEO didn't listen to actual engineers.
CF can make a fine pressure vessel it's just difficult to build and they require constant inspection that OceanGate didn't perform (and ignored evidence from their own monitoring system that the PV was responding differently to pressure after a hard bang it suffered when docking in one of the last few dives). SpaceX already has experience building the COPVs that Tesla is probably using and knows their limits pretty well though I don't want to be standing near one in a major crash either.
Different forces: expansive (Roadster) vs compressive (OceanGate). This makes some logical sense since fibers/strands/ropes are good for tensile strength vs. compressive forces.
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Some off piste skiers carry a carbon fiber canister with pressurized nitrogen gas, to inflate airbags from backpack in case of an avalanche. Carbon fiber is not that good with point pressure, but strong for evenly distributed loads.
At this point I look at wikipedia what happened to the submarine bamboozled mentioned. Ooops. Heh, the pressure of water is as evenly distributed as can be. Seems my background in watersports is not enough to moonlight in material engineering.
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SpaceX has been using carbon fiber pressure tanks for awhile. Rocket Labs has been using them too.
Professor Farnsworth: "Dear Lord! That's over 150 atmospheres of pressure!"
Fry: "How many atmospheres can the ship withstand?"
Professor Farnsworth: "Well, it's a space ship, so I'd say anywhere between zero and one."
You an actually see what the pressure vessel looks like in the recent Top Gear episode.
https://www.youtube.com/watch?v=JdkIOhFmbKI&t=1239
Doesn't look too big, tbh. They also explain some of the mechanics used to make it happen.
Well, yeah. The full thruster version only seats one, and is invite only for purchase, and is not street legal.
I'm wondering if it could even be sold to individuals given that the thrusters probably count as restricted military technology?
It's unclear what they'll be classified as and probably comes down to some negotiation and lawyering to decide how restricted they are. You can sell a lot of restricted hardware to US citizens that you can't sell internationally due to ITAR.
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Cold gas thrusters are big pressure vessels that vent through nozzles. You can build one in your garage and propel yourself on an office chair. People put them on drones for fun.
*Not street legal in 48/50 states
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It did say the flying version is a single seater.
The combination of a 250kg 5500psi pressure vessel, with a huge battery that generates a self fueling fire..... this thing will make for spectacular crashes
There is almost zero practical use for this technology. No one is going to drive around and pop a flip. The only possible use would be when some shit box car loses control and is about to head on crash a billionaire and the billionaire's car kickflips over the shit box and continues to drive while the shit box crashes behind him into flames.